title: Twin births, sex of children and maternal risk of endometrial cancer: a cohort study in Norway.
abstract: OBJECTIVE
  To explore whether twin births and sex of children influenced maternal risk of endometrial cancer, possibly with effect modification by age.
  
  
  METHODS
  Population-based prospective study.
  
  
  METHODS
  A total of 1,094,017 parous Norwegian women aged 30-74 years, including 3,356 endometrial cancer cases. Among the 27,158 mothers of twins, 101 cases occurred.
  
  
  METHODS
  Incidence rate ratios (IRR) with 95% confidence intervals (CI) were calculated in Poisson regression analyses of person-years at risk.
  
  
  RESULTS
  Women ever having experienced a twin birth had an overall higher risk of endometrial cancer than women with singleton births only (IRR=1.26, 95% CI=1.03-1.53). Women with twin boys appeared to be the main contributor to the overall elevated risk (IRR=1.57, 95% CI=1.15-2.14). The risk estimates for women with twin girls or sex-nonconcordant twins were close to unity (IRR of 1.09 and 1.12, respectively). However, age-specific analyses revealed an elevated risk also in women with twin girls, but only before age 55 years (IRR=1.92, 95% CI=1.27-2.89); a lower risk was seen at older ages (IRR=0.41, 95% CI=0.19-0.92). The risk estimates for twin boys and sex-nonconcordant twins were consistently observed across age groups. The effect modification by age was statistically significant (p=0.0024). No association was found with sex of children in singleton mothers.
  
  
  CONCLUSIONS
  Mothers of twin boys had a significantly higher risk of endometrial cancer than women with singleton births only, whereas women with twin girls had an elevated risk before age 55 years. No significant association was seen with sex-noncordant twins, neither overall nor within age groups.
authors.affiliation: Section for Epidemiology and Medical Statistics, Department of Public Health and Primary Health Care, University of Bergen, Norway. msega@uib.no
authors.forename: Grethe
authors.initials: G
authors.lastname: Albrektsen
authors.forename: Ivar
authors.initials: I
authors.lastname: Heuch
authors.forename: Steinar
authors.initials: S
authors.lastname: Thoresen
authors.forename: Gunnar
authors.initials: G
authors.lastname: Kvåle
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D015331:Cohort Studies; D016001:Confidence Intervals; D016889:Endometrial Neoplasms / Q000453:epidemiology* / Q000209:etiology*; D005260:Female; D006801:Humans; D016013:Likelihood Functions; D008297:Male; D008423:Maternal Age; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D016017:Odds Ratio; D010298:Parity; D011247:Pregnancy; D011272:Pregnancy, Multiple; D011446:Prospective Studies; D012042:Registries; D012307:Risk Factors; D012727:Sex Characteristics; D012737:Sex Factors; D014427:Twins